Defensio is much more of a subDPS rather than a tank or a healer, like Physician, but with less damage and heal overall. The hp-shield is quite weak and its mostly to protect yourself. As for if Defensio can replace another heal, it kinda depends on how the pt performs, but a downfall is that she doesnt have cure.

The only support defensio has to offer is aerial parry ex. Forget about beyond the wall, it's almost as bad as current charging howl if you compare it to guardian force.

you cant replace a healer anyway, unless you spend all your time spamming aerial parry which sounds stupid as heck and you don't have burst heal and you donìt have cure (the 45 skill is a self cure like relieve)

technically aerial parry ex should be 20% increased def for whole party and if you can keep it up by blocking on regular intervals, it will be permanent so that sounds nice but that is literally all defensio has to offer to the party, since if you have a guardian extra provoke is negligible.

So if that skill proves to be useful then defensio will be ok, otherwise ruina will do better because of more dmg

As a guardian, I have to say that the statement about the protection given by beyond the wall (Worse than guardian force) is widely mistaken. Just a small comparison between the stats gotten by applying the buff correctly:

Before:

After: (Actually, why the fork is the Mg def lower?)

Beyond the wall is incredibly powerful in terms of defending oneself (if bugs didnt exist), but it is really hard to aim this skill to get a whole party unless you are all very coordinated. In terms of area and reliability guardian wins, and if they are going to use the HP from the caster some day, Defensio's buff will be an interesting complement to guardian force. People tend to say defensio is no good because it is not perfect for a specific task, but if you like the class you can for sure specialize a bit, git gud, and support most roles quite decently by doing the following:

-Learning boss mechanics and patterns of attack: Effective blocking with air shove will create areas where characters get a defense buff and, on top of that, recover some HP. In some cases it can help someone resist a potentially mortal hit or regenerate damage dealt by fire and such (it is not that great tho, never rely on that as a proper heal).

-Reducing the enemy's power by 20% both magically and physically while enhancing your (or your teammates') defense and HP by 21/20% (depending on the presence or not of a tech ring).

-Provoking in the place of the main tank in case it somehow gets severe damage, dies or needs to evade a mechanic (auras in krudelis, for instance). Be careful with the placing of the marks though if the main provoker is fully functional, as it could disrupt their way of playing.

-Helping with damaging: Although defensio's damage is lower than ruina's it is still somehow decent, so you can help your teammates play with ease thanks to the nice defense values you get.

-Use the strike of the martyr omfg: People always complain this skill is suicidal, but it is freakin not. It needs to be used at the exact time with the appropriate buffs or it will (in this case) kill you. A simple setup would be BTW self and boss > Ulti and defense party buffs > BTW allies if possible > Leap over ex pull. If your HP pool is nice enough, with the stacked damage reduction (30% ulti, 20% leap pull, whatever % from ally buffs) and your own defenses, HP shields (BTW/ guardian force) and the reduced damage from the enemy it is almost guaranteed that the impact caused to super armor shields (like guardian force) and teammates HP will be greatly reduced. And on top of that, your team gets a +20% attack and Mattack. I see huge potential here.

Defensio is currently widely outclassed by guardian in tanking capabilities, but as any AGI based class, Machina gets a high crit rate together with a rather high base attack (and multi hit skills) which make it very fun to play and can fill a certain ammount of roles depending on your own skill and your party.